Amplitude modulation of optical signals plays an important role in various industrial sectors: e.g. communication, material processing. An extreme form of amplitude modulation is on/off modulation, which is the modulation format used in pulsed laser material processing and high-speed free space laser communication. Whereas in communication systems, the achievable modulation bandwidth is the crucial parameter, in laser based material processing it is rather the optical power handling capability.
The range and importance of laser based material processing in modern manufacturing is expanding at an impressive rate across many sectors in industry. Laser based material processing is inherently contact free. As such the problem of rapid wearing mechanical processing tools can be drastically reduced. The trend in pulsed laser material processing is to use short pulses with high peak power in order to improve the edge quality. The high laser beam intensity provided by short pulse laser technology results in the vaporization-dominated material removal rather than the melt-expulsion-dominated mechanisms using longer duration pulses. This produces less thermal and mechanical shocks, less peripheral heat flow, what leads to reduced heat affected zones (HAZ) and less burn formation and hence more precise material removal. Just as important the short pulse duration produces very high peak power. This high peak power allows the laser to process difficult materials. Due to the Q-switching mode, the peak power can be much higher than CW power lasers, meaning that much smaller lasers can be built to produce very high optical powers. Smaller lasers mean lower cost of ownership. Another advantage of such compact lasers is the possibility to mount them directly on robotic arms.
When short powerful laser pulses can be provided at a high repetition rate, precision laser based material processing can be drastically speeded up. A whole series of micro-machining applications can benefit from this: drilling or perforating of numerous small holes in industrial materials without charring the edges of the material, trimming applications. Heavy industrial applications such as for example welding, scribing, slotting, surface modifications of materials, surface removal, stripping, and medical applications such as for example surgery, dental, and dermatology applications also benefit from it.
The existing solutions for producing short pulses are the on/off switching of the direct driving power of the lasers or introducing an extra amplitude/phase/polarisation modulating device inside the laser cavity during the state of population inversion. The peak power and the efficiency of directly pulsed lasers are limited and their pulse repetition rates have an upper limit. Q-switching—the process whereby the pumping action of the laser is continuous so that a large population inversion is waiting for a moment when the beam is allowed for only a brief time to pass back and forth between the mirrors to achieve the laser action—can only be accomplished by introducing an additional modulation device. In Q-switching lasers the peak intensities can be tens to hundreds of times higher than in the direct drive schemes.
Another possibility to modulate the output of the laser is by switching the between the transverse modes of a laser.
A variety of optical modulation principles such as electro-optic, acousto-optic, magneto-optic and modulation schemes such as Fabry-Pérots modulators, Mach-Zehnder interferometers can be exploited inside the laser cavity to obtain Q-switching or high repetition rate pulse generation. In all existing solutions, whatever the physical interaction principle used, the area of the modulating device is at least as big as the area of the laser beam propagating inside the cavity. It is clear that in all these cases the cost of the modulating device is a function of the area of the laser beam.